Fig. 43. -- Skillett Creek, illustrating the points mentioned in the text.See larger image
The picturesque glens (Parfrey's and Dorward's) on the south face of the East bluff are the work of post-glacial streams. The preglacial valleys of this slope were obliterated by being filled during the glacial epoch.
The Wisconsin.—The preglacial course of the Wisconsin river is not known in detail, but it was certainly different from the course which the stream now follows. On PlateIthe relations of the present stream to the moraine (and former ice-front) may be seen.[10]As the ice approached it from the east, the preglacial valley within the area here under consideration was affected first by the overwash from the moraine, and later by the ice itself, from the latitude of Kilbourn City to Prairie du Sac.
It has already been stated that the ice probably dammed the river, and that a lake was formed above Kilbourn City, reaching east to the ice and west over the lowland tributary to the river, the water rising till it found an outlet, perhaps down to the Black river valley.
When the ice retreated, the old valley had been partly filled, and the lowest line of drainage did not everywhere correspond with it. Where the stream follows its old course, it flows through a wide capacious valley, but where it was displaced, it found a new course on the broad flat which bordered its preglacial course. Displacement of the stream occurred in the vicinity of Kilbourn City, and, forced to find a new line of flow west of its former course, the stream has cut a new channel in the sandstone. To this displacement of the river, and its subsequent cutting, we are indebted for the far-famed Dalles of the Wisconsin (p.69). But not all the present route of the river through the dalles has been followed throughout the entire postglacial history of the stream. In Fig.44, the depressiona,b,c, was formerly the course of the stream. The present course betweendandeis therefore the youngest portion of the valley, and from its lesser width is known as the "narrows." During high water in the spring, the river still sends part of its waters southward by the older and longer route.
The preglacial course of the Wisconsin south of the dalles has never been determined with certainty, but rational conjectures as to its position have been made.
The great gap in the main quartzite range, a part of which is occupied by Devil's lake, was a narrows in a preglacial valley. The only streams in the region sufficiently large to be thought of as competent toproduce such a gorge are the Baraboo and the Wisconsin. If the Baraboo was the stream which flowed through this gorge in preglacial time, the comparable narrows in the north quartzite range—the Lower narrows of the Baraboo—is to be accounted for. The stream which occupied one of these gorges probably occupied the other, for they are in every way comparable except in that one has been modified by glacial action, while the other has not.
Fig. 44. -- The Wisconsin valley near Kilbourn City.See larger image
The Baraboo river flows through a gorge—the Upper narrows—in the north quartzite range at Ablemans, nine miles west of Baraboo. This gorge is much narrower than either the Lower narrows or the Devil's lake gorge, suggesting the work of a lesser stream. It seems on the wholeprobable, as suggested by Irving,[11]that in preglacial time the Wisconsin river flowed south through what is now the Lower narrows of the Baraboo, thence through the Devil's lake gorge to its present valley to the south. If this be true, the Baraboo must at that time have joined this larger stream at some point east of the city of the same name.
Reference has already been made to the fact that the western part of the area here described is driftless, and the line marking the limit of ice advance has been defined. Beyond this line, gravel and sand, carried beyond the ice by water, extends some distance to the west. But a large area in the southwestern part of the state is essentially free from drift, though it is crossed by two belts of valley drift (valley trains) along the Wisconsin and Mississippi rivers.
The "driftless area" includes, besides the southwestern portion of Wisconsin, the adjoining corners of Minnesota, Iowa and Illinois. In the earlier epochs of the glacial period this area was completely surrounded by the ice, but in the last orWisconsin epochit was not surrounded, since the lobes did not come together south of it as in earlier times. (Compare PlateXXXVIIIand Fig.36.)
Various suggestions have been made in the attempt to explain the driftless area. The following is perhaps the most satisfactory:[12]
The adjacent highlands of the upper peninsula of Michigan, are bordered on the north by the capacious valley of Lake Superior leading off to the west, while to the east lies the valley of Lake Michigan leading to the south. These lake valleys were presumably not so broad and deep in preglacial times as now, though perhaps even then considerable valleys.
When the ice sheet, moving in a general southward direction from the Canadian territory, reached these valleys, they led off two great tongues or lobes of ice, the one to the south through the Lake Michigan depression, the other to the south of west through the Lake Superior trough. (Fig.36) The highland between the lake valleys conspired with the valleys to the same end. It acted as a wedge, diverting the ice to either side. It offered such resistance to the ice, that the thin and relatively feeble sheet which succeeded in surmounting it, did not advance far to the south before it was exhausted. On the other hand, the ice following the valleys of Lakes Superior and Michigan respectively, failed to come together south of the highland until the latitude of northern Iowa and Illinois was reached. The driftless area therefore lies south of the highlands, beyond the limit of the ice which surmounted it, and between the Superior and Michigan glacial lobes above their point of union. The great depressions, together with the intervening highland, are therefore believed to be responsible for the absence of glaciation in the driftless area.
The glaciated and unglaciated areas differ notably in (1) topography, (2) drainage, and (3) mantle rock.
1.Topography.—The driftless area has long been exposed to the processes of degradation. It has been cut into valleys and ridges by streams, and the ridges have been dissected into hills. The characteristic features of a topography fashioned by running water are such as to mark it clearly from surfaces fashioned by other agencies. Rivers end at the sea (or in lakes). Generally speaking, every point at the bottom of a river valley is higher than any other point in the bottom of the same valley nearer the sea, and lower than any other point correspondingly situated farther from the sea. This follows from the fact that rivers make their own valleys for the most part, and a river's course is necessarily downward. In a region of erosion topography therefore, tributary valleys lead down to their mains, secondary tributaries lead down to the first, and so on; or, to state the samething in reverse order, in every region where the surface configuration has been determined by rain and river erosion, every gully and every ravine descends to a valley. The smaller valleys descend to larger and lower ones, which in turn lead to those still larger and lower. The lowest valley of a system ends at the sea, so that the valley which joins the sea is the last member of the series of erosion channels of which the ravines and gullies are the first. It will thus be seen that all depressions in the surface, worn by rivers, lead to lower ones. The surface of a region sculptured by rivers is therefore marked by valleys, with intervening ridges and hills, the slopes of which descend to them. All topographic features are here determined by the water courses.
Fig. 45. -- Drainage in the driftless area. The absence of ponds and marshes is to be noted.See larger image
The relief features of the glaciated area, on the other hand, lack the systematic arrangement of those of the unglaciated territory, and stream valleys are not the controlling elements in the topography.
2.Drainage.—The surface of the driftless area is well drained. Ponds and lakes are essentially absent, except where streams have beenobstructed by human agency. The drainage of the drift-covered area, on the other hand, is usually imperfect. Marshes, ponds and lakes are of common occurrence. These types are shown by the accompanying maps, Figs.45and46, the one from the driftless area, the other from the drift-covered.
Fig. 46. -- Drainage in a glaciated region. Walworth and Waukesha counties, Wisconsin, showing abundance of marshes and lakes.See larger image
3.Mantle rock.—The unglaciated surface is overspread to an average depth of several feet by a mantle of soil and earth which has resulted from the decomposition of the underlying rock. This earthy material sometimes contains fragments and even large masses of rock like that beneath. These fragments and masses escaped disintegration because of their greater resistance while the surrounding rock was destroyed. This mantle rock grades from fine material at the surface down through coarser, until the solid rock is reached, the upper surface of the rock being often ill-defined (Fig.47). The thickness of the mantle isapproximately constant in like topographic situations where the underlying rock is uniform.
The residual soils are made up chiefly of the insoluble parts of the rock from which they are derived, the soluble parts having been removed in the process of disintegration.
Fig. 47. -- Section in a driftless area, showing relation of the mantle rock to the solid rock beneath.See larger image
With these residuary soils of the driftless area, the mantle rock of glaciated tracts is in sharp contrast. Here, as already pointed out, the material is diverse, having come from various formations and from widely separated sources. It contains the soluble as well as the insoluble parts of the rock from which it was derived. In it there is no suggestion of uniformity in thickness, no regular gradation from fine to coarse from the surface downward. The average thickness of the drift is also much greater than that of the residual earths. Further, the contact between the drift and the underlying rock surface is usually a definite surface. (Compare Figs.32and47.)
Since the ice melted from the region, the changes in its geography have been slight. Small lakes and ponds have been drained, the streams whose valleys had been partly filled, have been re-excavating them, and erosion has been going on at all points in the slow way in which it normally proceeds. The most striking example of postglacial erosion is the dalles of the Wisconsin, and even this is but a small gorge for so large a stream. The slight amount of erosion which has been accomplished since the drift was deposited, indicates that the last retreat of the ice, measured in terms of geology and geography, was very recent. It has been estimated at 7,000 to 10,000 years, though too great confidence is not to be placed in this, or any other numerical estimate of post-glacial time.
[1]Ripple marks are often seen on the surface of wind-blown sand, but the other features of this sandstone show that this was not its mode of accumulation.
[2]Irving: "The Baraboo Quartzite Ranges." Vol. II, Geology of Wisconsin, pp. 504-519. Van Hise: "Some Dynamic Phenomena Shown by the Baraboo Quartzite Ranges of Central Wisconsin." Jour. of Geol., Vol. I, pp. 347-355.
[3]A few hundred feet would suffice.
[4]Jour. of Geol., Vol. III (pp. 655-67).
[5]There is an admirable exposition of this subject in Gilbert's "Henry Mountains."
[6]It is not here asserted that these notches were as deep as now, in pre-Cambrian time. It is, however, certain that the quartzite was deeply eroded, previous to the deposition of the Potsdam sandstone.
[7]An account of loess in connection with the drift of the last glacial epoch is given in theJournal of Geology, Vol. IV, pp. 929-987. For a general account of loess, see Sixth Annual Report of U. S. Geological Survey.
[8]Chamberlin: Geology of Wisconsin, Vol. 1.
[9]The moraine line on the map represents the crest of the marginal ridge rather than its outer limit, which is slightly nearer the lake margin. Stratified drift of the nature of overwash also intervenes at points between the moraine and the lake border.
[10]The preglacial course was probably east of the present in the vicinity of Kilbourn City.
[11]Irving. Geology of Wisconsin, Vol. II.
[12]Chamberlin and Irving. Geology of Wisconsin, Vols. I and II.